Let me start by saying that I love using Scala and consider it a net positive but stating that anyone who can't grok Scala is a poor developer just absolutely smacks of arrogance. Every tool has drawbacks.
1) Documentation for the standard Scala library is absolute crap. The JDK is much better at not only documenting methods in a reasonable way but also giving you common usage patterns for classes/packages. I really don't like having to piece together the method signatures of a class to figure out the intended usage pattern.
2) This might be a problem in IntelliJ but I can't figure out a good way of getting the debugger to work well. Sometimes it'll catch breakpoints other times it won't.
3)Lack of maturity in libraries. A perfect example is Scala's built in XML support. It works for 90% of what you want to do but finds good ways to waste your time, especially if you're dealing with namespaces.
4) Obscure syntax and abuse of the single argument method call syntax. What does :+ mean to anyone? What's the difference between +++ and ++ or :: and :::. Scala is a dyslexic's nightmare and I don't agree with such heavy use of symbolic shorthand.
Again I love Scala and have used it write some heavily parallel code with minimal effort thanks to the functional nature of Scala but I feel like the learning curve is a lot harder than it should be.